Field of the Invention
The present invention concerns a method for carrying out calibration measurements in an MR system, as well as an MR system for implementing such a method.
Description of the Prior Art
In MR systems, in order to acquire an MR image, the MR signals must be acquired using system parameters such as the resonance frequency, transmitter reference amplitude, or the polarization field B0, have to be adjusted to the test subject in order to be able to obtain meaningful MR images of the test subject. In order to determine the parameters for these system components, a stand-alone calibration step is carried out in each case. This usually involves a specific stand-alone MR measurement and an evaluation provided therefor. Within such an MR measurement, which is carried out in a system control computer, the time schedule can be set and carried out in the sub-microsecond range. This is relevant, for example, when the calibration step is formed by multiple sub-steps that are dependent on one another and, for example, executed in repeated iterations that require a specific time delay between them. Each MR measurement is prepared in a computer, the “host-computer”, and after an unknown and varying time delay, the MR measurement is initiated by the system control computer after the computer has informed the system control computer that and how the measurements are intended to be carried out.
FIG. 1 shows the procedure for such a calibration measurement in the prior art. In a step S1, a preparation step is carried out on a computer 100 of the MR system, which is operated by a user of the MR system in order to control the MR system. In this step, the determination of an MR center frequency for adjusting the MR system to a test subject is prepared. When adjusting the center frequency, the maximum of the frequency or the resonance frequency of water, known as the Larmor frequency for the test subject in the MR system is determined. Knowledge of the exact frequency is important, for example, when determining the frequency of an RF pulse that is radiated by the MR system onto the test subject. This preliminary or preparation step S1 includes, inter alia, loading program files into the main memory of the computer, allocating storage space in the program memory, etc. After preparing for this calibration step by running the program file that is intended to carry out the frequency calibration, there is a time delay, shown in step S1a, before the computer 100 commands the system control computer 200 to carry out the calibration of the frequency. Communication between the computer 100 and the system control computer 200 is necessary for this before the frequency calibration can finally be carried out in step S2. There is a further time delay S2a, which is needed to transmit the information to the system control computer and to prepare the measurement itself on the system control computer, until the measurement can finally be initiated in step S2. In step S2b the computer is informed in step S2 that the frequency calibration has been completed. Before the computer 100 can carry out the preparation for a further calibration step to adjust the transmitter voltage in step S3, there is a further time delay, labelled as step S3a. As in step S1, the preparation includes loading the respective program sections into the main memory, etc. Before this calibration of the voltage can be carried out in step S4, there are again time delays for the required communication and preparation, shown by step S3b and step S4a. The computer is again informed of the end of this calibration step and, before a polarization field B0 can be adjusted to the test subject step S5, there are again time delays S4b and S5a. After the preparation for this adjustment has been completed in step S5, there are time delays S5b and S6a, before the B0 field can be adjusted to the test subject in step S6, for example, by switching the currents in shim coils provided for this purpose, which compensate for any magnetic field inhomogeneities so that the polarization field B0 is as homogeneous as possible across the measurement field. After completion of step S6, the time delays S6b and S7a are required for communication between the computer 100 and the system control computer 200. Until the currents in the shim coils have finally stabilized to homogenize the polarization field B0, there is a waiting time in step S7. Following this, after a time delay S8a in step S8, a further frequency calibration can be prepared, which can again be initiated by the system control computer in step S9 after the time delays S8b and S9, the time delay in steps S8b and S9b again being due to communication between units 100 and 200. After completion of the frequency adjustment in step S9, there follows in step S9b and in S10a information on the end of the calibration steps, until the system is ready to start the actual MR measurement, after completion of step S10a. 
The individual preparation phases and time delays that result inter alia from communication between the two units extend the overall time required for the calibration measurements.